[Qgis-user] Re: OSGB coordinates to WGS84 lat/lon problem

[Etienne Tourigny]

datum shift transformations have been fixed in qgis-1.8, which may
explain this.  This has been addressed in this list (and also
qgis-dev) in the last months

Etienne

On Thu, May 10, 2012 at 4:04 PM, Nick Hopton <nhopton at gmail.com> wrote:
>
> Andy Harfoot wrote
>>
>> Have you tried the convertor here:
>> http://gps.ordnancesurvey.co.uk/convert.asp
>> to see whether the higher accuracy coordinates obtained from QGIS match
>> those obtained using OSTN02?
>>
>
> For the record, the Survey gives the following (ETRS89) lat/lon coordinates
> for the points:
>
> Inkpen: 51.3524757 -1.4650872
> Hodmore: 51.4984893 -1.0170283
> Hindhead: 51.1154179 -0.7157938
>
> I think ETRS89 has drifted about half a metre away from WGS84 at the present
> time, but even so the transformations done by Lisboa under Ubuntu look very
> good.
>
>
>
>> It looks as though Proj4 can use grid based transformations like OSTN02
>> through the use of gsb files. Could it be that this is the case in the
>> higher accuracy situation? I find gsb files at the following location on
>> my Win7 QGIS 1.7.4 install:
>> C:\OSGeo4W\share\proj
>>
>
> At present by default QGIS doesn't use a grid-shift file for EPSG:27700. The
> datum-shift is carried out by the +towgs84 part of the Proj parameter
> string.
>
>
>
>> I can't see any files in this directory suggesting they are the OSTN02
>> grid though, and I can reproduce your 'Vista' results as follows,
>> suggesting that my QGIS is using the lower accuracy seven parameter
>> transform.
>>
>
> It's not using a grid-shift file. But it's interesting that you get the same
> poor transformations that I get with 1.7.4 under Visa. Lisboa RC1 under
> Ubuntu produces much better transformations (for me) but I don't know why
> this is the case.
>
> Nick.
>
>
> --
> View this message in context: http://osgeo-org.1560.n6.nabble.com/OSGB-coordinates-to-WGS84-lat-lon-problem-tp4965339p4966416.html
> Sent from the Quantum GIS - User mailing list archive at Nabble.com.
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